This invention relates generally to gas turbine engines and methods for oil cooling in such engines.
Gas turbine engines are commonly provided with a circulating oil system for lubricating and cooling various engine components such as bearings, gearboxes, and the like. In operation the oil absorbs a substantial amount of heat that must be rejected to the environment in order to maintain the oil at acceptable temperatures. Commonly, the oil is circulated through an oil-to-fuel heat exchanger where heat from the oil is rejected to the fuel, which acts as a heat sink. The fuel is subsequently injected into the engine's combustor and burned.
In many operating conditions, aircraft gas turbine engines have more oil heat load than heat sink from the fuel which will be burnt in the engine. The typical solution to this is to either cool engine fuel or engine oil with engine fan air, or to pump fuel through the oil-to-fuel heat exchanger at a higher rate than required for combustion, with the excess fuel flow being recirculated from the engine back to the aircraft fuel tanks. Low-bypass military turbofan engines have too many fan stages (typically three) to make fan air cooling a viable solution, because the fan duct discharge air is too hot. Therefore, tank recirculation is used.
FIG. 1 depicts an example of a prior art aircraft gas turbine engine 10 with a fuel tank recirculation system. The engine 10 has a fan 12, a high pressure compressor 14, a combustor 16, a high pressure turbine 18, and a low pressure turbine 20, all arranged in a serial, axial flow relationship. The engine 10 is operable to generate a core flow of exhaust gases as well as a bypass flow in a conventional manner. In the illustrated example, the engine 10 is a low-bypass turbofan in which a portion of the flow from the fan 12 is directed around the core in a bypass duct 22. The bypass flow and the core flow both exit into an afterburner duct 24 which has an afterburner flameholder 26 disposed at its upstream end.
A fuel-to-oil heat exchanger 28 is coupled to the lubrication system 30 of the engine 10. A feed pump 32 pumps fuel from the tanks 34 of the aircraft (not shown) through the fuel-to-oil heat exchanger 28 where it absorbs heat from the oil. The fuel then passes downstream where it is metered into the combustor 16 and burned. In many cases the heat load required to be rejected from the oil is greater than the heat sink capacity of the fuel at the required fuel flow for the engine operating condition. For example, this can occur when the oil is at a high temperature and the fuel flow is low (e.g. flight idle). Accordingly, to get sufficient cooling, fuel is supplied to the fuel-to-oil heat exchanger 28 at the required rate for cooling, then the excess above that needed for engine operation is routed back to the tanks 34.
During ground idle the fuel in the tanks 34 may become very hot and it may become necessary to use ground support equipment to cool the fuel. During flight, tank fuel temperature increases at an ever increasing rate as the tanks 34 become near empty. Furthermore, this raises the temperature of the fuel as subsequently supplied to the engine 10. The engine 10 may have limits on the acceptable input fuel temperature. For example, the fuel may be needed at a relatively low temperature for cooling a full authority digital engine control (FADEC) or other electronics.